Glucocorticoid regulation of hepatic S-adenosylmethionine synthetase gene expression.

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The effects of glucocorticoids on the regulation of rat liver S-adenosylmethionine synthetase were studied in vivo and in two culture systems. Livers from adrenalectomized animals were examined for enzyme activity, immunoreactive protein, and messenger RNA (mRNA) content. All three parameters showed a similar trend, i.e. they decreased 3-fold after adrenalectomy and increased over the control values upon triamcinolone replacement. These results suggested that glucocorticoid regulation of hepatic S-adenosylmethionine synthetase was mediated at the mRNA level. Triamcinolone and dexamethasone increased S-adenosylmethionine synthetase mRNA content in a time- and dose-dependent manner in both rat hepatoma H35 cells and primary cultures of adult rat hepatocytes. The kinetics of mRNA induction were identical in both culture systems, indicating that the hormone-mediated response is independent of the differentiated state of the cell. Insulin blocked the inducing effect of glucocorticoids on S-adenosylmethionine synthetase mRNA in a dose-dependent manner. On the other hand, the triamcinolone-dependent increase in mRNA levels was completely abolished by treatment with actinomycin D, whereas cycloheximide did not affect this response. The transcription rate of the gene, as measured by run-on assay, increased 3-fold after hormone addition. Transient transfections of H35 cells with 1.4 kilobases of the 5'-flanking region of the hepatic S-adenosylmethionine synthetase gene fused to a luciferase reporter gene showed that promoter activity is also increased 3-fold after triamcinolone treatment, suggesting that this promoter region contains the sequence elements necessary to confer glucocorticoid responsiveness. In addition to the transcriptional control of the hepatic S-adenosylmethionine synthetase gene, our results suggest that glucocorticoids may be acting at a posttranscriptional level.